The present invention relates to medical procedures such as cardiac ablation and to devices and components useful in these and other uses.
Contraction or xe2x80x9cbeatingxe2x80x9d of the heart is controlled by electrical impulses generated at nodes within the heart and transmitted along conductive pathways extending within the wall of the heart. Certain diseases of the heart known as cardiac arrhythmias involve abnormal generation or conduction of the electrical impulses. One such arrhythmia is atrial fibrillation or xe2x80x9cAF.xe2x80x9d Certain cardiac arrhythmias can be treated by deliberately damaging the tissue of the cardiac wall along a path crossing a route of abnormal conduction. This causes formation of a scar extending along the path where disruption occurred. The scar blocks conduction of the electrical impulses. Such a scar can be created by conventional surgery, but this entails all of the risks and expense associated with cardiac surgery. Another approach, described in Swartz et al., U.S. Pat. No. 5,575,766, is to introduce a catheter bearing a localized energy emitter such as an electrode for application of radio frequency (xe2x80x9cRFxe2x80x9d) energy at its distal tip into a heart chamber, such as the right or left atrium of the heart in the case of atrial fibrillation. The physician then moves the catheter so that the tip, and the localized emitter traces the desired path. In AF, the desired path typically is a closed loop encircling the openings or ostia of the pulmonary veins. RF energy applied through the electrode heats the tissue to a degree sufficient to cause death of the normal tissue and its replacement by scar tissue. Heating to this degree is referred to herein as xe2x80x9cablation.xe2x80x9d Typically, heating to about 60-80xc2x0 C. is sufficient. Tracing a precise path along the interior of a chamber in the heart of a living subject with the tip of a catheter involves inherent practical difficulties. Although curved guide wires can be placed within the catheter so that the catheter tip will tend to follow the guide wire as the physician moves it, the process is still difficult.
Swanson et al., U.S. Pat. No. 5,582,609 describes an elongated catheter having numerous RF electrodes disposed along its length in a distal region adjacent the tip. This distal region can be formed into a curved, loop-like configuration and manipulated so that the electrodes lie along the desired path, whereupon RF energy is applied so as to ablate cardiac tissue. In a variant of this approach, the electrodes are mounted on a structure which opens to form a ring-like configuration. Even with these structures, however, it is difficult to assure the desired placement of the RF electrodes. Lesh, U.S. Pat. No. 5,971,983 describes an elongated catheter which is equipped with similar RF electrodes distributed over its distal region, and uses guide wires to position the distal region in place against the wall of the heart. Although this patent mentions an xe2x80x9cultrasonic element such as an ultrasound crystal elementxe2x80x9d along with numerous other devices as theoretically applicable to cardiac tissue ablation, it offers no structure for an elongated ultrasonic ablating device.
As described in Lesh, International Publication WO 99/02096, the abnormal conduction routes in AF typically extend from the wall of the heart along the pulmonary veins. Therefore, AF can be treated by ablating tissue in a ring around each pulmonary vein at the juncture between the pulmonary vein and the heart. As described in the ""096 publication, such ablation can be performed by threading a catheter having a thermal ablation element at its distal tip into the heart so that the tip is lodged within the appropriate pulmonary vein. The catheter may bear a balloon which is inflated within the vein and which holds the catheter in place. The ablating element is then actuated so as to apply heat in a region surrounding the ablating element. In certain embodiments taught in the ""096 publication, the ablating element includes a radio frequency (xe2x80x9cRFxe2x80x9d) emitting element which is carried on the surface of the balloon. Ablation of the pulmonary vein using RF energy can create a rough, disrupted surface on the interior of the vein. This or other factors can lead to stenosis of the pulmonary vein or thrombosis, i.e., formation of blood clots.
Other embodiments described in the ""096 publication disclose the use of ultrasonic transducers. The preferred ultrasonic transducer illustrated in the ""096 publication is a rigid ceramic piezoelectric element disposed on a catheter surrounded by a balloon. When the balloon is inflated, the piezoelectric element remains remote from the wall of the pulmonary vein. The piezoelectric element can be actuated to apply sonic energy through a fluid contained in the balloon, thereby heating the ring of vein wall tissue surrounding the balloon. As a further alternative, the ""096 publication shows an ultrasonic emitter in the form of a hollow concave disk. The ""096 publication suggests that such an emitter can be physically rotated around the axis of a catheter so as to ablate a ring-like zone. These transducers have numerous drawbacks even for use in ablation of a vein wall and are not adapted for ablation of the wall of the cardiac chamber.
Ultrasonic heating such as high intensity focused ultrasound (HIFU) is utilized for certain therapeutic applications. As disclosed in commonly assigned International Application PCT/US98/1062, published as International Publication WO/98/52465 the disclosure which is hereby incorporated by reference herein, HIFU heating typically is conducted using an ultrasonic emitter having an array of transducers. The transducers are actuated with a drive signal so as to emit ultrasonic waves. The relative phasing of the waves is controlled by the physical configuration of the array and the phasing of the drive signal. These factors are selected so that the ultrasonic waves tend to reinforce one another constructively at a focal location. Tissue at the focal location is heated to a greater extent than tissue at other locations. As described, for example in commonly assigned U.S. patent application Ser. No. 09/496,988, filed Feb. 2, 2000 and in commonly assigned U.S. patent application Ser. No. 09/532,614, the disclosures of which are also incorporated by reference herein, HIFU may be applied by transducer arrays such as arrays of polymeric piezoelectric transducers. These arrays can be mounted on a probe such as a catheter which can be introduced into the body as, for example, within the vascular system or into a cavernous internal organ. The ""988 application discloses certain transducer arrays which can be deformed so as to vary the placement of the focal location.
Despite all of these efforts in the art, there have been needs for further improvements in the devices and methods used to apply thermal energy to the cardiac wall for treatment of atrial fibrillation, particularly the need to tightly control the zone of damage to cardiac tissue in order to minimize collateral damage to neighboring tissues. There have been corresponding needs for further improvement in the devices and methods used to apply energy to other organs of the body for thermal treatment.
The present invention addresses these needs.
One aspect of the present invention provides apparatus for applying energy within the body of a living subject. Apparatus according to this aspect of the invention preferably includes a probe having a proximal end and a distal end adapted for insertion into the body of the patient. For example, the probe may include one or more catheters. An ultrasonic emitter is provided adjacent the distal end of the probe.
The apparatus according to this aspect of the invention also includes an expansible structure mounted on said probe adjacent the distal end thereof. The expansible structure has a collapsed condition and an expanded condition. The expansible structure includes a reflector balloon having an interior space. The ultrasonic emitter is disposed outside of the interior space of the reflector balloon. The reflector balloon has an active region juxtaposed with the emitter when the expansible structure is in the expanded condition, so that ultrasonic energy emitted by the emitter will impinge on the active region from outside of the reflector balloon. Thus, when the reflector balloon is inflated with a gas and a liquid is present outside of the reflector balloon, the gas within the reflector balloon and the liquid will form a reflective interface at the active region. Ultrasonic energy emitted by the will be reflected from the active region towards tissue of the subject adjacent the expansible structure.
Most preferably, the expansible structure further includes a structural balloon having an interior space encompassing the ultrasonic emitter. The structural balloon and the reflector balloon preferably are contiguous in the active region. Most preferably, the two balloons share a common wall at the active region. In operation, the structural balloon may be inflated with a liquid, so that the reflective interface is formed at the common wall by the liquid in the structural balloon and the gas in the reflector balloon. For example, the probe may have separate conduits communicating with the interior spaces of the two balloons.
The structural balloon desirably has a transmissive wall adapted to overlie a wall of an internal organ of the subject when said expansible structure is in said expanded condition. In this condition, the active region is configured so that ultrasonic energy will be reflected from the active region, through the interior space of the structural balloon to the transmissive wall. The ultrasonic energy will pass through the transmissive wall to the wall of the internal organ. The liquid in the structural balloon desirably is an aqueous liquid or other liquid having acoustic impedance close to that of the body tissue to minimize reflection at the interface with the tissue.
Most preferably, the ultrasonic emitter is substantially in the form of a surface of revolution about a central axis extending in forward and rearward directions, and the active region is also substantially in the form of a surface of revolution about the central axis when the expansible structure is in its expanded condition. Thus, the active region can direct energy into an annular treatment region in the form of an annular or loop-like path extending along the wall of the organ around the central axis.
Most preferably, the active region is adapted to focus the ultrasonic energy reflected at the active region into a loop like focal region which extends along the path but which has area smaller than the active region. In this manner, the ultrasonic energy is concentrated to provide a high energy density, so that the applied ultrasonic energy provides rapid heating along the entire path. For example, the active region may be a surface of revolution of a parabola about the central axis. As further explained below, such a surface will focus energy from a simple cylindrical ultrasonic emitter into an annular focal region.
Preferred apparatus according to this aspect of the invention can be used, for example, to ablate tissue of the wall of the atrium encircling the ostium of a pulmonary vein. Tissue along an annular path of about 25-30 mm diameter can be ablated so as to form a full transmural lesion, extending entirely through the atrial wall to provide a full conduction block, in a few minutes or less, using about 15 Watts of ultrasonic power. Even shorter ablation times can be achieved using higher power.
The ability to treat tissue along an annular path of large diameter is particularly advantageous inasmuch as it allows formation of the lesion in the cardiac wall, rather than in the pulmonary vein itself. This minimizes stenosis of the pulmonary vein, which is a significant drawback of pulmonary vein ablation. However, the expansible structure and transducer can be extremely compact when in the collapsed condition. Preferably, the expansible structure and transducer are about 4 mm or less in diameter when in the collapsed condition, and can be placed into the heart by threading the probe through the vascular system.
The apparatus desirably is arranged to place the focal region within the wall of the organ, at a desired depth from the surface of the wall. Ultrasonic ablation using a focal region within the wall minimizes formation of rough scar tissue at the wall surface, and thus minimizes thrombogenesis when the apparatus is used to treat the heart. Placement of the focal region within the wall also promotes rapid heating.
Preferably, the structural balloon has properties similar to those of a balloon regarded as a xe2x80x9cnoncompliantxe2x80x9d balloon in the arts of balloon angioplasty and related arts. Such a balloon is quite rigid when inflated, and is not deformed appreciably by physiologic pressures such as the pressure of blood. Typically, such balloons are inflated to significant pressure, typically several atmospheres or more. In the preferred embodiments, the structural balloon maintains the active region in a precise shape to assure sharp focusing, and helps to position the expansible structure with respect to the heart or other organ to be treated so as to provide precise placement of the focal region.
A related aspect of the invention provides apparatus for applying energy within a subject. The apparatus according to this aspect of the invention provides an expansible structure for insertion into the body of the subject. The expansible structure includes a reflector having an active region. The expansible structure has a collapsed condition and an expanded condition. The apparatus also includes an energy emitter operative to apply energy while the expansible structure is in the expanded condition and disposed within the body of the subject so that the applied energy is directed onto the active region of the reflector and reflected by the active region of the reflector towards the tissue of the subject adjacent the reflector. Most preferably, the expansible structure is operative to focus the energy as well as redirect it. The focusing action preferably is provided by the active region of the reflector, although, as explained below, other expansible elements such as an inflatable lens can be used to provide focusing. The ability to provide both focusing and reflective redirection in the expansible structure contributes to the action of the apparatus as discussed above. In this aspect of the invention, the energy emitter most preferably is an ultrasonic emitter, although other forms of energy may be applied.
A related aspect of the invention provides methods of applying energy to a living subject including the steps of positioning an expansible structure including a reflector, such as the structures discussed above with reference to the apparatus, within or adjacent to the body of the subject and bringing the expansible structure to an expanded condition. The method further includes the step of directing energy onto an active region of the reflector so that energy is reflected from the active region and directed onto a desired region of the subject. Most preferably, the expansible structure focuses the energy in addition to redirecting it. The expansible structure may be disposed within the body of the subject in or adjacent to an organ and the energy may be directed onto a desired region of the wall of the organ as, for example, onto the interior wall of a heart chamber. As discussed above in connection with the apparatus, the energy may be sonic energy such as ultrasonic waves. In one particularly preferred method, the expansible structure is positioned within a chamber of the heart and the energy is directed onto a treatment region extending along an elongated path on the interior wall of the heart as, for example, along a path at least partially surrounding and desirably entirely surrounding the ostium of a blood vessel communicating with the heart chamber, such as the ostium of a pulmonary vein. Desirably, energy is directed onto the entire path simultaneously. Because the entire path can be ablated or otherwise treated simultaneously, there is no need to reposition the probe carrying the expansible structure during the procedure.
The preferred apparatus and methods according to the foregoing aspects of the invention are simple and inherently reliable. Merely by way of example, the most preferred apparatus can employ simple ultrasonic transducers with a single piezoelectric element, and balloon structures which can be fabricated with known techniques.
A further aspect of the present invention provides an acoustic reflector for directing ultrasonic energy comprising a first or structural balloon and a second or reflector balloon, the balloons being inflatable and deflatable. The balloons are contiguous with one another at an active region at least when the balloons are in an inflated condition. The structure desirably includes a first port communicating with the interior of the first balloon and a second port communicating with the interior of the second balloon, so that the first and second balloons can be filled with different fluids having different acoustic impedances so as to form a reflective interface at the active region. A structure in accordance with this aspect of the invention can be used as a component of the apparatus described above, or in other applications.
Another aspect of the invention provides techniques of monitoring and controlling cardiac ablation procedures as discussed above. Still other aspects of the invention provide features which facilitate orderly collapse of the balloon structures after use, so as to facilitate withdrawal of the apparatus after the procedure.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.